Recent advances in the fabrication of ultratransparent materials have demonstrated that fibers are a promising transmission medium for optical communication systems. These light transmitting fibers, which are referred to as optical waveguides generally consist of a transparent core surrounded by a layer of transparent cladding material having a refractive index which is lower than that of the core. A low loss optical waveguide and a method of making the same are disclosed in U.S. Pat. No. 3,659,915. The waveguide described in that patent comprises a cladding layer of fused silica and a core of fused silica doped with one or more materials that selectively increase the index of refraction of the core above that of the cladding.
If such fibers are to be used in optical communication systems, means must be provided for quickly and conveniently connecting sections of fiber together in the course of their use. Such means must cause light to be coupled from one fiber to another with little loss of signal, i.e., with low insertion loss. When two optical waveguides are joined end-to-end, insertion loss can result from center-to-center mismatch, angular misalignment of the fiber axes, fiber-to-fiber separation, rough end finish and Fresnel reflections. For example, consider the insertion loss which may occur when two fused silica optical waveguides of the type disclosed in the aforementioned U.S. Pat. No. 3,659,915 are joined end-to-end. For this illustration, it is assumed that the fibers have a diameter of 5 mils and a cladding thickness of 1 mil. If misregistration of the centers of the two fibers is the only loss mechanism, a loss of about 1 db occurs if the centers thereof are separated by 0.5 mil. For reasonably low insertion losses to be achieved, the centers of the fiber endfaces must therefore be registered within 0.5 mil. Futhermore, due to the low numerical aperture of present optical waveguides, the fiber axes must be substantially aligned, i.e., they must be aligned to within 3.degree. to keep insertion losses less than about 1 db, assuming no other loss mechanisms are operative. Since light diverges from the fiber axis as it radiates from a fiber, some light is lost if the endfaces of the fibers are separated; therefore, the endfaces of the fibers should be maintained in virtual contact. Fiber-to-fiber separation also implies an insertion loss due to Fresnel reflections at the two glass-air interfaces. This loss, however, can be substantially eliminated by disposing between the fiber endfaces a layer of index matching material such as an oil or other fluid having a refractive index of about 1.5.
Minimizing these insertion losses becomes especially difficult when cables or bundles of fibers are to be connected. The axes of all fibers in both cables should be substantially parallel, the endfaces of the fibers in one cable should be touching the endfaces of the fibers in the other cable, and the endface of a fiber in one cable should be substantially centered with respect to that of a corresponding fiber in the other cable. A connector exhibiting the aforementioned features is disclosed in said related application Ser. No. 498,329. Briefly, the connector of that application comprises a first member of resilient material having at least one elongated V-shaped groove in a surface thereof for receiving a pair of fibers to be connected, the two walls forming the V-shaped groove being substantially symmetrical about a first plane and the axes of the pair of fibers being disposed in the first plane. A second member of resilient material having at least one elongated elevated portion is aligned with the groove and is adapted to contact that surface of the fibers opposite the groove. Means are provided for applying a force to the first and second members in a direction perpendicular to the fiber axes and in the first plane, the force tending to bring them together and deform them against the adjacent surfaces of the fibers, thereby simultaneously aligning and mechanically securing the fibers. Although the connector disclosed in said related application can be employed to connect the fibers of two cables, it is extremely difficult to cut the fibers of each cable to the precise length necessary to provide contact between the endfaces of all fibers of one cable with the corresponding endfaces of the fibers of the other cable.